Hey Rob, (Sorry long answer coming 
)
this one has really got me thinking!
Ok, found out a couple of things:
Correct, they don’t show in the Assets panel. Here’s a link to Forum question that asked a similar question:
Correct.
The setting is revealed with sites created prior to a certain date.
Generating responsive images for older sites
If you created your site before September 14th, 2016, you’ll need to walk through a quick process to generate responsive images on your site. Next time you open the Designer, you’ll see a modal asking you to start this process.
Found a Blog Post that has more info when this feature was released:
No. It looks like everything is already being looked after.
So in summary, (after all this information overload!!!)
Webflow automatically creates and serves a ‘Responsive’ image that is used with the Image Element.
Best practice is to ALWAYS compress images BEFORE uploading, whether to Assets panel or to CMS.
Hope that helps, sorry if it’s overkill 
I was interested to find out more myself.
Regards,
Keiran